To construct disaster-resistant optical path protection in an optical network, it is desirable to install a standby transmission path sufficiently away from an active transmission path so that faults do not occur in any of the active and standby transmission paths due to the same cause. A multi-level modulation system, such as 16-quadrature amplitude modulation (QAM) and 32-QAM, is used as a modulation system of an optical signal in recent years to expand the transmission capacity. The higher the degree of multi-level in the multi-level modulation system, the higher the transmission capacity. However, the transmittable distance is reduced. An example of related art is disclosed in Japanese Laid-open Patent Publication No. 2007-180793.
When the standby transmission path is installed away from the active transmission path, the standby transmission path becomes longer than the active transmission path, and there is a problem that it is difficult to reserve an optical signal to noise ratio (OSNR) for maintaining sufficient transmission quality in the standby transmission path. For example, the higher the degree of multi-level in the multi-level modulation system, the higher the desirable OSNR for maintaining sufficient transmission quality. However, the OSNR is reduced when the distance of the standby transmission path is extended.
In this regard, the OSNR for maintaining sufficient transmission quality may be reserved if the degree of multi-level in the multi-level modulation system is reduced when the active transmission path is switched to the standby transmission path. However, the process of changing the degree of multi-level takes a long time (several seconds to several tens of seconds, for example) due to, for example, the change in the setting of digital signal processor (DSP), and it is difficult to switch the transmission path within a desirable time (within 50 milliseconds, for example).
In view of the foregoing, it is desirable to provide a transmission apparatus, a transmission system, and a transmission method that may reduce the time required to switch the transmission path.